Auxiliary Power Units (APUs) are critical for the safe and reliable operation of modern aircraft, providing electrical power and compressed air during ground operations and in-flight emergencies. As APUs operate worldwide, they are exposed to harsh conditions such as sand ingestion leading to compressor deterioration and APU performance degradation. The objective of this thesis, conducted in collaboration with EPCOR, was to investigate whether Computational Fluid Dynamics (CFD) can be used to predict compressor performance degradation and its impact on overall APU performance to improve APU condition monitoring and predictive maintenance strategies.

In this study, the centrifugal compressor of a Pratt & Whitney APS5000, as used in the Boeing 787, was reverse engineered using 3D scans of the impeller and diffuser. These geometries were reconstructed and implemented in a CFD model that was validated against a pass-off test measurement. Through a literature study, it was concluded that the main compressor deterioration effects are increases in impeller tip clearance and impeller and diffuser surface roughness. The impact of these effects on compressor efficiency, pressure ratio, and flow capacity was simulated and incorporated into a Gas turbine Simulation Program (GSP) model to assess the resulting APU performance degradation by evaluating changes in Exhaust Gas Temperature (EGT), fuel flow, and compressor pressure ratio.

The results show that increased surface roughness and tip clearance both lead to reductions in compressor efficiency, pressure ratio and flow capacity, which translate into higher exhaust gas temperatures and increased fuel flow at the APU system level. Plotting the reduction in pressure ratio, increase in EGT and increase in fuel flow as a function of compressor efficiency deterioration and flow capacity deterioration, a compressor deterioration map was made. This map is overlaid with simulated points of varying surface roughness and/or tip clearance serving as a decision tool to aid in root cause determination during APU disassembly.

Although the absolute accuracy of the results is limited by assumptions in geometry reconstruction, turbulence modeling, and validation data, the study provides an indication of the relative reduction in APU system performance and demonstrates a working proof of concept in the form of a deterioration map. Therefore, it is concluded that compressor CFD with gas turbine simulation offers a viable approach to assess compressor deterioration effects and their impact on APU performance, thus enhancing APU condition monitoring and supporting root cause determination in a maintenance environment.